Topic 8Properties of Water

GEOL 2503

Introduction to Oceanography

H20

• Englishman, Sir Humphrey Davey discovered the formula for water: H2O

• 3 Atoms: two hydrogen & one oxygen• H+ H+ O2-

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Polar Molecule

• Positive end – Hydrogens on tips of “V”

• Negative end – Oxygen on other end

• Molecules attract one another

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Properties of Water

• Thermal (Heat Capacity)• Cohesion• Surface Tension• Viscosity• Transmission of light, sound

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Thermal Properties of Water

• Phase changes

• Heat capacity

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Changes of StateSolid - ice

Liquid – liquid water

Gas -water vaporWater is the only common substance to exist in all three states of matter at normal Earth surface temperatures

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January 8, 2010; 8 AM

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January 8, 2010; 2 PM

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To change a state of matter

• Need time to add or remove heat

• Need time for the hydrogen bonds to break

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Pure Water

• 100 % water—nothing else• No suspended particles• No dissolved substances including gases

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State Change Temperatures

• Pure water melts/freezes at 0° C (32 ° F)• Pure water boils/condenses at 100 ° C (212

° F)

– [at standard air pressure]

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For example:

• Take solid water (ice)• Add heat• Temperature rises above freezing point• Ice melts• Forms liquid water

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Another example:

• Start with liquid water• Add heat• Temperature rises• Water evaporates to form gaseous water

(water vapor)

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Also works in reverse:

• Start with water vapor (gas)• Remove heat (cool)• Temperature falls• Water vapor condenses to form liquid water• Remove more heat• Freezing• Solid water (ice) forms

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Heat is not the same as temperature

What is Heat?

• Heat is energy • Measured in calories• A calorie is the amount of heat needed to

raise the temperature of 1 gram of water by 1° C

• [Kcal (1000 calories) = 1 food calorie]

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Remember:

• It takes 1 calorie of energy (heat) to raise the temperature of 1 gram of liquid water by 1° C

• Also, if you remove 1 calorie of energy the temperature of 1 gram of liquid water will then drop 1° C

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But look at state (phases) changes:

• To change 1 gram of ice at 0° C to liquid water at 0° C

• Requires 80 calories per gram not 1 calorie

• Called the latent heat of fusion

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And:

• To change 1 gram of liquid water at 100° C to gas (water vapor) at 100° C

• Requires 540 calories per gram not 1 calorie

• Called the latent heat of vaporization

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Why?

• Not changing temperature• Breaking chemical bonds• That takes energy (heat)

• Works in reverse by taking heat away (chemical bonds are formed)

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Solid Liquid Gas

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A--hydrogen bondB--oxygen atomC--hydrogen atom

D--positively charged side of the water moleculeE--negatively charged side of the water molecule

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State Change Terminology

• Solid to liquid: melting• Liquid to gas: evaporation• Gas to liquid: condensation• Liquid to solid: freezing• Solid to gas: sublimation• Gas to solid: deposition

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Heat Capacity

• Is defined as the quantity of heat required to produce a unit change of temperature in a unit mass of that material

• Heat capacity of water is 1 calorie per gram per degree Celsius

• Heat capacity of water is higher than most other liquids due to hydrogen bonding

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Heat capacities:

Liquid water = 1 cal/g/ºC

Ice = 0.5 cal/g/ºC

Water vapor = 0.5 cal/g/ºC

How many calories does it take to raise the temperature of 1 gram of water from -100 ºC to +150 ºC ?

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time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16water 73 78 78 78 78 78 78 78 78 79 79 79 80 81 81 81 81dry sand 72 76 77 79 81 82 83 84 84 84 84 85 85 87 87 87 87damp sand 75 78 78 79 80 80 80 81 81 82 82 83 83 83 84 85 85

Heat Capacity

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minutes

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era

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waterdry sanddamp sand

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High heat capacity helps move heat around the Earth, moderating climate

• Water evaporates from oceans, absorbing heat from oceans

• Winds move water vapor • Water returns to liquid form by

precipitation, adding heat to the atmosphere

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Range of normal Earth Temperaturescontrolled by latent heat

• Deserts +50° C (122° F)• Antarctica -50° (-58° F)

– Land range 100° C (180° F)

• Tropical oceans +28° C (82° F)• Polar oceans -2° C (28° F)

– Ocean range 30° C (54° F)

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Land versus Ocean Temperatures

• Land:– +50° C (122° F) to -50° (-58° F)– Land range 100° C (180° F)

• Water:– +28° C (82° F) to -2° C (28° F)– Ocean range 30° C (54° F)

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“Evaporation is a cooling process”

• You’ve probably heard that phrase• To evaporate water takes 540 calories of

heat per gram• Essentially absorbing heat• Example—cooling our bodies (removing

heat energy) by evaporation of perspiration

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Heat Capacity Summary:

• Day/night change of water temperature is very small

• Helps redistribute heat around Earth• Helps stabilize Earth’s surface temperatures

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Cohesion

• Water has more structure than other liquids due to the hydrogen bonds - hold water together

• Water molecules stay close together due to polarity

• Makes water relatively sticky

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Surface Tension

• Water molecules form a film because of sticky nature of water

• Paper clip floats, water striders• Overfill glass of water• High surface tension is important

for waves• Decrease temperature increases

surface tension and vice versa37

Viscosity

• Resistance to flow• High viscosity means high resistance to

flow• Affected by temperature—“slow as

molasses in January”

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Density

• Less dense floats on denser liquids• Example: Ocean water is denser than fresh

water so fresh floats on salt water• Defined: mass per unit volume of a

substance• Measured in grams per unit centimeter

cubed

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What is mass?

• Mass is the amount of matter present• It is not the same as weight• Weight changes depending on gravity• Mass remains constant• Think of astronauts—normal weight on

Earth, weightless in space, 1/6th of Earth weight on moon, but their mass is constant

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Density

• Pure water: D = 1.0000 g/cm3

– At 4° C (39.2° F)

• Sea water has salt dissolved in it, so it is denser than pure water

• Sea water: D = 1.0278 g/cm3

– At 4° C (39.2° F)

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Coke Can Density Experiment

• Can of regular Coke sinks in water• Same size can of Diet Coke floats

• WHY?

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Effect of Temperature on Density

• As heat is added, molecules move faster and move farther apart

• End up with less mass per unit volume• Therefore, as temperature increases, density

decreases

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Less dense substances float on denser substances

• Dry pine wood floats on water• Oil floats on water• Fresh water floats on salt water• Warm water floats on cold water

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Ice and Density

• As pure water cools, density increases• Maximum D at 4° C ( D = 1 g/cm3)• Below 4° C solid crystal ice structure starts

to form and molecules are pushed apart• Same mass but occupying larger volume so

lower density• Ice is 10 % less dense than water at 4° C• Ice floats

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Note how water molecules are farther apart than in liquid water47

Effect of Salt on Density

• Dissolved materials increase density of water (more mass per unit volume)

• Pure water: D = 1.000 g/cm3

• Fresh water: D ≈ 1.000 g/cm3 • Sea water: D = 1.0278 g/cm

– [at 4° C]

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Pressure• Increases with depth• For every 10 m (33 ft.) in depth

the pressure increases by 1 atmosphere

• 1 atmosphere = 14.7 pounds per square inch

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Water = Universal Solvent

• Essentially everything dissolves in water

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Light• Visible light is narrow band of electromagnetic

spectrum• Visible light is broken into rainbow ROY G BIV• Red absorbed within 33 ft. in ocean• Blue-green light spectrum goes deeper Objects seen in natural color at the surface Objects in deeper water appear dark because

illuminated by blue light

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Violet

Blue

Green

Yellow

Orange

Red

The Electromagnetic SpectrumGamma Rays X-Rays

Ultraviolet

Visible Light

Infrared RaysMicrowavesRadarShort Wave FM TVBroadcast Band

Long Radio Waves52

Visible Light & Seawater

• 60% is absorbed in 1st meter• 80% is absorbed by 10 m (33 ft)• 99% is absorbed by 150 m (500 ft)• 100% is absorbed by 1000 m (3300 ft)

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Why is the Sea Blue?

• Color = reflected light• Red end of spectrum absorbed• Blue-green end reflected back• We see the reflected blue end of the

spectrum• ROY G BIV

Absorbed Reflected back

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Transmission of light in water is measured using a Secchi disk

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The Secchi Disk measures water clarity. Simply lower the disk and record water depth at which you can no longer see it. Very low-tech, but effective measure.

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Sound

• Travels faster in water than air• Velocity 1500 m/s (5000 ft/s)• In dry air 334 m/s (1100 ft/s)• Speed increases if

– Temperature increases– Pressure increases– Salt increases

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PDR and DSL

• PDR = Precision depth recorder• Uses narrow sound beam traces bottom

while ship is in motion• Echo Finders = fish finders• DSL = Deep Scattering Layer • Organisms/fish found by depth recorders

(creates false bottom on depth recorder)

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SONAR(Sound Navigation & Ranging)

• Underwater location system• Locates objects and finds targets• Echo reflected from target• Sound shadows formed• Sound changes speed as passes through

different densities of water

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SOFAR Sound Channel

• Sound is transmitted more effectively and rapidly in water than it is in air

• Sound velocity in the ocean is about 1500 m/s, or about 4-5 times that in air

• Sound is focused into the SOFAR channel because of its low velocity region

• Sound is transmitted best through this channel -- important for submarines and cetaceans

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SOFAR Channel Experiments63